1. Field of the Invention
This invention relates to a method and apparatus for detecting a candidate area for a microcalcification pattern embedded in an object image.
2. Description of the Related Art
Image processing, such as gradation processing or processing in a frequency domain, has heretofore been performed on an image signal, which represents an image and has been obtained with one of various image acquiring techniques, such that a visible image having good image quality can be reproduced and used as an effective tool in, particularly, the accurate and efficient diagnosis of an illness. Particularly, in the field of medical images, such as radiation images of human bodies serving as objects, it is necessary for specialists, such as doctors, to make an accurate diagnosis of an illness or an injury of the patient in accordance with the obtained image. Therefore, it is essential to perform the image processing in order that a visible image having good image quality can be reproduced and used as an effective tool in the accurate and efficient diagnosis of an illness.
In such image processing, the processing is often performed on the entire area of the image. Alternatively, in cases where the purpose of examination or diagnosis is clear to a certain extent, enhancement processing is often performed selectively on a desired image area, which is adapted to the purpose of examination or diagnosis.
Ordinarily, when an image area to be processed is to be selected, the person, who views the radiation image, views the original image before being processed and manually selects the image area to be processed. However, there is the risk that the selected image area or the specified image range will vary, depending upon the level of the experience or the image understanding capability of the person, who views the radiation image, and the selection cannot be made objectively.
For example, in cases where a radiation image has been recorded for the examination of breast cancer, it is necessary to find a microcalcification pattern, which is one of features of a cancerous area, from the radiation image. However, the range of the microcalcification pattern cannot always be specified accurately. Therefore, there is a strong demand for techniques for accurately detecting an abnormal pattern, such as a microcalcification pattern, without depending upon the skill of the person, who views the radiation image.
In order to satisfy the demand described above, extensive research has been conducted to make a computer aided diagnosis of medical images (CADM), such that a calcification pattern candidate is capable of being detected automatically by use of processing with computers. As one of the CADM techniques, morphological filter processing has been proposed. [The morphological filter processing is proposed in, for example, “Extraction of Microcalcifications Using Morphological Filter with Multiple Structuring Elements,” Kobatake, et al., Transactions of The Institute of Electronics, Information, and Communication Engineers of Japan, D-II, Vol. J75-D-II, No. 7, pp. 1170–1176, July 1992 (hereinbelow referred to as Literature 1); and “Fundamentals of Morphology and Its Application to Mammogram Processing,” Medical Imaging Technology, Vol. 12, No. 1, January 1994 (hereinbelow referred to as Literature 2).]
The applicant proposed various abnormal pattern candidate detecting processing systems (computer aided medical image diagnosing apparatuses), wherein abnormal pattern candidate detecting processing is performed for automatically detecting a calcification pattern, which is one of forms of abnormal patterns and is embedded in a mammogram (a mamma image for diagnosis), a chest image, or the like, by use of a morphological filter, and an image obtained by superposing an image, which has been obtained by performing enhancement processing, image size enlargement processing, or the like, on a region of an abnormal pattern candidate having been detected, upon a certain area of an entire area image, such that the image has good image quality and can serve as an effective tool in the efficient and accurate diagnosis, an index value in the abnormal pattern candidate detecting processing, or the degree of certainty of detection is outputted. The proposed abnormal pattern candidate detecting processing systems are disclosed in, for example, Japanese Unexamined Patent Publication Nos. 8(1996)-263648, 8(1996)-272961, 8(1996)-294479, and 9(1997)-106448.
With the abnormal pattern candidate detecting processing in which the morphological filter is utilized, an image area, in which the image density fluctuates within a range spatially narrower than a multi-structure element (i.e., a mask) having a size in accordance with the size of an abnormal pattern to be detected, is detected as an abnormal pattern candidate by use of the multi-structure element. (The abnormal pattern candidate detecting processing in which the morphological filter is utilized will hereinbelow be referred to as the morphological filter processing.) With the morphological filter processing, a candidate for a microcalcification pattern, which is one of characteristic forms of breast cancers, is capable of being detected automatically. (The microcalcification pattern is one of forms of abnormal patterns.) The morphological filter processing has the features in that, for example, (1) it is efficient for extracting a calcification pattern itself, (2) it is not affected by complicated background information, and (3) the extracted calcification pattern does not become distorted. Specifically, for example, in cases where the morphological filter processing is utilized for a mammogram, a linear structure, or the like, is capable of being kept unremoved from the mamma image, and only fine structures, such as microcalcifications and noise, are capable of being removed from the mamma image. Thus the morphological filter processing is advantageous over differentiation processing, which is an ordinary technique for enhancing a specific structure element, in that the detection of the calcification pattern is capable of being performed such that the geometrical information concerning the size, the shape, and the image density distribution of the calcification pattern is kept more accurately.
The morphological filter processing is an image processing technique appropriate for extraction and recognition of a feature concerning the shape in an image. As described above, the morphological filter processing has been studied particularly as a technique efficient for detecting the microcalcification pattern, which is one of characteristic forms of breast cancers. However, the image to be processed with the morphological filter processing is not limited to the microcalcification pattern in the mammogram, and the morphological filter processing is applicable to any kind of image, in which the size and the shape of a specific image area (i.e., an abnormal pattern, or the like) to be detected are known previously.
However, in cases where the microcalcification pattern candidate is to be detected with the morphological filter processing, the problems occur in that, theoretically, a linear component and a structure not longer than the structure element length, which have a direction that does not coincide with any of the directions of the structure elements constituting the multi-structure element, cannot be removed. (Both the linear component and the structure not longer than the structure element length, which are described above, will hereinbelow be referred to as the linear structure area.) The problems described above cause erroneous detection to occur during the detection of the microcalcification pattern. Also, a microcalcification pattern, which is unsharp or is constituted over a plurality of pixels, and a non-calcification pattern, which has a size identical with the size of the calcification pattern, cannot always be removed with the morphological filter processing. (Both the microcalcification pattern, which is unsharp or is constituted over a plurality of pixels, and the non-calcification pattern, which has a size identical with the size of the calcification pattern, will hereinbelow be referred to as the coarse structure area.)
Therefore, in cases where image enhancement processing is performed on the image, which has been obtained from the morphological filter processing, the problems occur in that, besides the calcification pattern, the fine structures of the linear structure area and the coarse structure area, which could not be removed with the morphological filter processing, are also enhanced and appear as noise. As a result, an image, which is hard to see, is obtained.
In order for the problems described above to be solved, techniques have been proposed, wherein the image, which has been obtained from the morphological filter processing, is subtracted from the original image, a fine structure image, which illustrates a fine structure area in the image, is thereby formed, threshold value processing is performed on the fine structure image, and a microcalcification pattern is thereby detected. The proposed techniques are disclosed in, for example, Literature 1 described above, Literature 2 described above, and Japanese Unexamined Patent Publication No. 9(1997)-106448. However, the proposed techniques are not perfectly satisfactory.